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Catalysis and chemical mechanisms of calcite dissolution in seawater.

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Title: Catalysis and chemical mechanisms of calcite dissolution in seawater.
Authors: Subhas, Adam V.1 asubhas@gps.caltech.edu, Adkins, Jess F.1, Rollins, Nick E.2, Naviaux, John1, Erez, Jonathan3, Berelson, William M.2
Source: Proceedings of the National Academy of Sciences of the United States of America. 8/1/2017, Vol. 114 Issue 31, p8175-8180. 6p.
Subjects: Sea water analysis, Calcite analysis, Secondary ion mass spectrometry, Crystal structure, Dissolution (Chemistry)
Abstract: Near-equilibrium calcite dissolution in seawater contributes significantly to the regulation of atmospheric CO2 on 1,000-y timescales. Despite many studies on far-from-equilibrium dissolution, little is known about the detailed mechanisms responsible for calcite dissolution in seawater. In this paper, we dissolve 13C-labeled calcites in natural seawater. We show that the time-evolving enrichment of δ13C in solution is a direct measure of both dissolution and precipitation reactions across a large range of saturation states. Secondary Ion Mass Spectrometer profiles into the 13C-labeled solids confirm the presence of precipitated material even in undersaturated conditions. The close balance of precipitation and dissolution near equilibrium can alter the chemical composition of calcite deeper than one monolayer into the crystal. This balance of dissolution--precipitation shifts significantly toward a dissolution-dominated mechanism below about Ω = 0:7. Finally, we show that the enzyme carbonic anhydrase (CA) increases the dissolution rate across all saturation states, and the effect is most pronounced close to equilibrium. This finding suggests that the rate of hydration of CO2 is a rate-limiting step for calcite dissolution in seawater. We then interpret our dissolution data in a framework that incorporates both solution chemistry and geometric constraints on the calcite solid. Near equilibrium, this framework demonstrates a lowered free energy barrier at the solid--solution interface in the presence of CA. This framework also indicates a significant change in dissolution mechanism at Ω = 0:7, which we interpret as the onset of homogeneous etch pit nucleation. [ABSTRACT FROM AUTHOR]
Copyright of Proceedings of the National Academy of Sciences of the United States of America is the property of National Academy of Sciences and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
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An: 124445666
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PubTypeId: academicJournal
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  Data: Catalysis and chemical mechanisms of calcite dissolution in seawater.
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  Data: <searchLink fieldCode="AR" term="%22Subhas%2C+Adam+V%2E%22">Subhas, Adam V.</searchLink><relatesTo>1</relatesTo><i> asubhas@gps.caltech.edu</i><br /><searchLink fieldCode="AR" term="%22Adkins%2C+Jess+F%2E%22">Adkins, Jess F.</searchLink><relatesTo>1</relatesTo><br /><searchLink fieldCode="AR" term="%22Rollins%2C+Nick+E%2E%22">Rollins, Nick E.</searchLink><relatesTo>2</relatesTo><br /><searchLink fieldCode="AR" term="%22Naviaux%2C+John%22">Naviaux, John</searchLink><relatesTo>1</relatesTo><br /><searchLink fieldCode="AR" term="%22Erez%2C+Jonathan%22">Erez, Jonathan</searchLink><relatesTo>3</relatesTo><br /><searchLink fieldCode="AR" term="%22Berelson%2C+William+M%2E%22">Berelson, William M.</searchLink><relatesTo>2</relatesTo>
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  Data: <searchLink fieldCode="DE" term="%22Sea+water+analysis%22">Sea water analysis</searchLink><br /><searchLink fieldCode="DE" term="%22Calcite+analysis%22">Calcite analysis</searchLink><br /><searchLink fieldCode="DE" term="%22Secondary+ion+mass+spectrometry%22">Secondary ion mass spectrometry</searchLink><br /><searchLink fieldCode="DE" term="%22Crystal+structure%22">Crystal structure</searchLink><br /><searchLink fieldCode="DE" term="%22Dissolution+%28Chemistry%29%22">Dissolution (Chemistry)</searchLink>
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  Data: Near-equilibrium calcite dissolution in seawater contributes significantly to the regulation of atmospheric CO2 on 1,000-y timescales. Despite many studies on far-from-equilibrium dissolution, little is known about the detailed mechanisms responsible for calcite dissolution in seawater. In this paper, we dissolve 13C-labeled calcites in natural seawater. We show that the time-evolving enrichment of δ13C in solution is a direct measure of both dissolution and precipitation reactions across a large range of saturation states. Secondary Ion Mass Spectrometer profiles into the 13C-labeled solids confirm the presence of precipitated material even in undersaturated conditions. The close balance of precipitation and dissolution near equilibrium can alter the chemical composition of calcite deeper than one monolayer into the crystal. This balance of dissolution--precipitation shifts significantly toward a dissolution-dominated mechanism below about Ω = 0:7. Finally, we show that the enzyme carbonic anhydrase (CA) increases the dissolution rate across all saturation states, and the effect is most pronounced close to equilibrium. This finding suggests that the rate of hydration of CO2 is a rate-limiting step for calcite dissolution in seawater. We then interpret our dissolution data in a framework that incorporates both solution chemistry and geometric constraints on the calcite solid. Near equilibrium, this framework demonstrates a lowered free energy barrier at the solid--solution interface in the presence of CA. This framework also indicates a significant change in dissolution mechanism at Ω = 0:7, which we interpret as the onset of homogeneous etch pit nucleation. [ABSTRACT FROM AUTHOR]
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  Data: <i>Copyright of Proceedings of the National Academy of Sciences of the United States of America is the property of National Academy of Sciences and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)
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        Value: 10.1073/pnas.1703604114
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        Text: English
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        PageCount: 6
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      – SubjectFull: Sea water analysis
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      – SubjectFull: Calcite analysis
        Type: general
      – SubjectFull: Secondary ion mass spectrometry
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      – SubjectFull: Crystal structure
        Type: general
      – SubjectFull: Dissolution (Chemistry)
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      – TitleFull: Catalysis and chemical mechanisms of calcite dissolution in seawater.
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              Text: 8/1/2017
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